When graphical image data is produced by a computing device, or host, it is commonly produced in pixels comprising values for the levels of red (R), green (G), and blue (B). This is known as RGB. The pixel data can then be transmitted as separate R, G, and B values, or more usually, the pixel data can be converted to luma (Y) and chroma (a, (3) values where luma indicates the luminescence of the pixel and chroma indicates its colour.
Luma is calculated as follows:Y=k(R+G+B)
Where k is a constant to appropriately scale the Y value
The two chroma values comprise parts of the original RGB value as follows:α=aR+bG+cB β=a′R+b′G+c′B 
where a, b, c and a′, b′ and c′ are constants. A simple transform often used makes a=0, b=1, c=1 and a′=1, b′=1, c′=0, resulting in:α=G+B β=G+R 
The constants are always pre-programmed and are not changed to adapt to different circumstances. Naturally, this leads to a preponderance of green, which is corrected using, so-called, gamma correction that takes place before the pixels are displayed on a screen.
There are other standard transforms for different colour spaces available by which the balance of the RGB values can be set by constant weightings before they are coded into the luma and chroma values, although this method is the most common in conventional use. As mentioned above, these constants are pre-programmed for specific fixed colour spaces and are not changed to adapt to different circumstances. In any case, conversion back into RGB relies on the use of a known colour space in which the constants used are not changed.